Multi-chip type image sensor

ABSTRACT

A multi-chip type image sensor includes, a base, a plurality of sensor chips arranged on the base so as to be staggered so that adjacent sensor chips among the sensor chips partially overlap with each other in the direction perpendicular to a direction in which the sensor chips are arranged. The sensor chips are fastened to the base by an adhesive for use in die bonding. Each of the sensor chips has a plurality of light receiving elements arranged into a line at a predetermined pixel pitch Pd. Each of the sensor chip contains silicon. Therminal pins are supported on the base and electrically coupled to the sensor chips. The following formula is satisfied: 
     
         Pc=Pd×[1+(Tc-Ta)(α.sub.B -αSi.sub.)] 
    
     where Tc [°C.] is a curing temperature of the adhesive, Ta [°C.] is an environment temperature at which the image sensor is to be used, α Si  is a thermal expansion coefficient of each of the sensor chips comprising silicon, α B  is a thermal expansion coefficient of the base, and Pc is a pixel pitch at which the light receiving elements are arranged in each of the sensor chips before the adhesive is cured by die bonding.

BACKGROUND OF THE INVENTION

The present invention generally relates to a multi-chip type imagesensor, and more particularly to a full-size contact type image sensorwhich includes a plurality of integrated circuit image sensor chips.Further, the present invention is concerned with a method of producingthe multi-chip type image sensor.

Recently, in the field of image scanners, there has been considerableactivity in the development of a full-size contact type image sensor inorder to reduce the size of an optical system. There has been proposedan image sensor which employs an integrated circuit image sensor chipusing a single silicon wafer. However, there is a limit in length ofsuch an image sensor due to the size of the used silicon wafer.Therefore, an image sensor using a single silicon wafer is not suitablefor constructing a full-size image sensor. In order to form aone-dimensional image sensor capable of reading a document of anordinary size, it is required to arrange a plurality of integratedcircuit image sensor chips on the same base. Such a sensor is called amulti-chip type image sensor. A conventional multi-chip type imagesensor has been disclosed in "CCD CONTACT IMAGE SENSOR", ICS87-55.

In a conventional multi-chip type image sensor as proposed, a pluralityof integrated circuit image sensor chips are arranged into a line.Alternatively, a plurality of chips are arranged so that adjacent chipspartially overlap with each other. Each of the integrated circuit imagesensor chips is formed of silicon and has a plurality of light receivingelements which are disposed at the same pitch (interval). Each of lightreceiving elements is photosensitive and generates an electric signalcorresponding to light irradiated onto it. The integrated circuit imagesensor chips are fastened on a single alumina base by die bonding, forexample.

The following is known die bonding methods. Various parametersindicative of characteristics of the following die bonding methods areshown in respective tables.

                  TABLE 1                                                         ______________________________________                                        (Eutectic Bonding)                                                            ______________________________________                                        Bonding material  Au--Si, Au--Ge or Au--Sn                                    Metallization for die                                                                           Unnecessary or                                              back surface      Au evaporation                                              Working temperature                                                                             320-450° C.                                          Working atmosphere                                                                              Inactive atmosphere or                                                        reducing atmosphere                                         Ohmic property    Good                                                        Heat dissipation  Good                                                        Wire bonding adaptability                                                                       Yes                                                         Furnace seal adaptability                                                                       Yes                                                         Mass productivity Good                                                        Cost              High                                                        ______________________________________                                    

                  TABLE 2                                                         ______________________________________                                        (Solder Bonding)                                                              ______________________________________                                        Bonding material                                                                              Pb-based solder or                                                            Sn-based solder                                               Metallization for die                                                                         Ni, Ni--Au or Cr--Ni--Au etc.                                 back surface                                                                  Working temperature                                                                           250-350° C.                                            Working atmosphere                                                                            Inactive atmosphere or                                                        reducing atmosphere                                           Ohmic property  Good                                                          Heat dissipation                                                                              Good                                                          Wire bonding adaptability                                                                     No only for                                                                   thermocompression bonding                                     Furnace seal adaptability                                                                     Yes                                                           Mass productivity                                                                             Good                                                          Cost            Medium                                                        ______________________________________                                    

                  TABLE 3                                                         ______________________________________                                        (Resin Bonding)                                                               ______________________________________                                        Bonding material  Ag + epoxy, Ag + polyimide,                                                   or Ag + silicon                                             Metallization for die                                                                           Unnecessary                                                 back surface                                                                  Working temperature                                                                             Room temp. (curing occurs                                                     at 150-200° C.)                                      Working atmosphere                                                                              Air or inactive atmosphere                                  Ohmic property    Unstable                                                    Heat dissipation  Poor                                                        Wire bonding adaptability                                                                       Yes                                                         Furnace seal adaptability                                                                       Yes only for polyimide                                      system                                                                        Mass productivity Excellent                                                   Cost              Low                                                         ______________________________________                                    

                  TABLE 4                                                         ______________________________________                                        (Glass Bonding)                                                               ______________________________________                                        Bonding material   PbO--B.sub.2 O.sub.3 -based glass                          Metallization for die                                                                            Unnecessary                                                back surface                                                                  Working temperature                                                                              420-600° C.                                         Working atmosphere Air                                                        Ohmic property     Poor                                                       Heat dissipation   Poor                                                       Wire bonding adaptability                                                                        Yes                                                        Furnace seal adaptability                                                                        Yes                                                        ______________________________________                                    

The above-indicated die bonding methods require high workingtemperatures. In the present state of the field, the resin bondingmethod is better than the other bonding methods in view of reliability,work efficiency and mass productivity.

In the resin bonding method, a heat curing type adhesive for die bondingis coated on a base to a thickness of about tens of microns with a widthcorresponding to that of sensor chip at room temperature by a dispenseprocess, a stamp process or a screen printing process. Then, a pluralityof sensor chips are located on the coated adhesive and pressed againstthe base. Thereafter, the sensor chips are subjected to a batch processby use of an oven in which the sensor chips are subjected to a heatcuring process for one to two hours at a heating temperature between 120and 250° C.

The following conductive adhesive agents for use in die bonding areavailable in a market. Various parameters indicative of properties ofthe following conductive adhesive agents are shown in respective tables.

                  TABLE 5                                                         ______________________________________                                        (Chemitight CT212 manufactured by                                             Toshiba Chemical Corp)                                                        ______________________________________                                        State (Compounding ratio)                                                                            one-liquid type                                        Solvent                available                                              Composition (filler/resin)                                                                           Ag/epoxy                                               Curing condition (temp./time)                                                                        200° C./1 hr                                    Volume resistivity (Ω · cm)                                                           0.6 × 10.sup.-4                                  Heat conductivity (cal/cm · sec. °C.)                                                6 × 10.sup.-3                                    Extracted impurity (Cl.sup.- |Na.sup.+)                                                     5|5                                           ______________________________________                                    

                  TABLE 6                                                         ______________________________________                                        (EN-4000 manufactured by Hitachi Chemical Corp.)                              ______________________________________                                        State (Compounding ratio)                                                                            one-liquid type                                        Solvent                available                                              Composition (filler/resin)                                                                           Ag/epoxy                                               Curing condition (temp./time)                                                                        175° C./1 hr                                    Volume resistivity (Ω · cm)                                                           2 × 10.sup.-4                                    Heat conductivity (cal/cm · sec. °C.)                                                0.6 × 10.sup.-3                                  Extracted impurity (Cl.sup.- |Na.sup.+)                                                     10|10                                         ______________________________________                                    

                  TABLE 7                                                         ______________________________________                                        (EN-4070X-13 manufactured by Hitachi Chemical Corp.)                          ______________________________________                                        State (Compounding ratio)                                                                            one-liquid type                                        Solvent                none                                                   Composition (filler/resin)                                                                           Ag/epoxy                                               Curing condition (temp./time)                                                                        150° C./1 hr to                                                        250° C./40 sec                                  Volume resistivity (Ω · cm)                                                           3.3 × 10.sup.-4                                  Heat conductivity (cal/cm · sec. °C.)                                                --                                                     Extracted impurity (Cl.sup.- |Na.sup.+)                                                     10|10                                         ______________________________________                                    

                  TABLE 8                                                         ______________________________________                                        (CRM-1038 manufactured by Sumitomo Bakelite Corp.)                            ______________________________________                                        State (Compounding ratio)                                                                          one-liquid type                                          Solvent              none                                                     Composition (filler/resin)                                                                         Ag/epoxy                                                 Curing condition (temp./time)                                                                      200° C./1 hr ˜ 170° C.                                    /20 sec + 350° C.                                                      /20 sec                                                  Volume resistivity (Ω · cm)                                                         2 × 10.sup.-4                                      Heat conductivity (cal/cm · sec. °C.)                                              3 × 10.sup.-3                                      Extracted impurity (Cl.sup.- |Na.sup.+)                                                   10|10                                           ______________________________________                                    

                  TABLE 9                                                         ______________________________________                                        (CRM-1058 manufactured by Sumitomo Bakelite Corp.)                            ______________________________________                                        State (Compounding ratio)                                                                            one-liquid type                                        Solvent                available                                              Composition (filler/resin)                                                                           Ag/polyimide                                           Curing condition (temp./time)                                                                        150° C./1 hr ˜                                                   250° C./1 hr                                    Volume resistivity (Ω · cm)                                                           2 × 10.sup.-4                                    Heat conductivity (cal/cm · sec. °C.)                                                --                                                     Extracted impurity (Cl.sup.- |Na.sup.+)                                                     10|20                                         ______________________________________                                    

                  TABLE 10                                                        ______________________________________                                        (Ablebond 84-1 LMI manufactured by Ablestic Corp.)                            ______________________________________                                        State (Compounding ratio)                                                                            one-liquid type                                        Solvent                none                                                   Composition (filler/resin)                                                                           Ag/epoxy                                               Curing condition (temp./time)                                                                        150° C./1 hr                                    volume resistivity (Ω · cm)                                                           2 × 10.sup.-4                                    Heat conductivity (cal/cm · sec. °C.)                                                4.5 × 10.sup.-3                                  Extracted impurity (Cl.sup.- |Na.sup.+)                                                     10|10                                         ______________________________________                                    

                  TABLE 11                                                        ______________________________________                                        (Ablebond 71-1 LMI manufactured by Ablestic Corp.)                            ______________________________________                                        State (Compounding ratio)                                                                            one-liquid type                                        Solvent                available                                              Composition (filler/resin)                                                                           Ag/polyimide                                           Curing condition (temp./time)                                                                        150° C./30 min ˜                                                 275° C./30 min                                  volume resistivity (Ω · cm)                                                           2 × 10.sup.-4                                    Heat conductivity (cal/cm · sec. °C.)                                                --                                                     Extracted impurity (Cl.sup.- |Na.sup.+)                                                     10|5                                          ______________________________________                                    

                  TABLE 12                                                        ______________________________________                                        (EPO-TEK H-20ELC manufactured by                                              Epoxy Technology Corp.)                                                       ______________________________________                                        State (Compounding ratio)                                                                            two-liquid type (1:1)                                  Solvent                none                                                   Composition (filler/resin)                                                                           Ag/epoxy                                               Curing condition (temp./time)                                                                        120° C./15 min                                  volume resistivity (Ω · cm)                                                           3 × 10.sup.-4                                    Heat conductivity (cal/cm · sec. °C.)                                                4 × 10.sup.-3                                    Extracted impurity (Cl.sup.- |Na.sup.+)                                                     30|--                                         ______________________________________                                    

                  TABLE 13                                                        ______________________________________                                        (EPO-TEK H35-175M manufactured by                                             Epoxy Technology Corp.)                                                       ______________________________________                                        State (Compounding ratio)                                                                            one-liquid type                                        Solvent                none                                                   Composition (filler/resin)                                                                           Ag/epoxy                                               Curing condition (temp./time)                                                                        180° C./1 hr                                    volume resistivity (Ω · cm)                                                           2 × 10.sup.-4                                    Heat conductivity (cal/cm · sec. °C.)                                                --                                                     Extracted impurity (Cl.sup.- |Na.sup.+)                                                     10|10                                         ______________________________________                                    

                  TABLE 14                                                        ______________________________________                                        (Du Pont 4621D manufactured by Du Pont)                                       ______________________________________                                        State (Compounding ratio)                                                                            one-liquid type                                        Solvent                available                                              Composition (filler/resin)                                                                           Ag/epoxy                                               Curing condition (temp./time)                                                                        175° C./1 hr                                    volume resistivity (Ω · cm)                                                           4 × 10.sup.-4                                    Heat conductivity (cal/cm · sec. °C.)                                                --                                                     Extracted impurity (Cl.sup.- |Na.sup.+)                                                     20|10                                         ______________________________________                                    

                  TABLE 15                                                        ______________________________________                                        (C-990 manufactured by Amicon Corp.)                                          ______________________________________                                        State (Compounding ratio)                                                                            one-liquid type                                        Solvent                none                                                   Composition (filler/resin)                                                                           Ag/epoxy                                               Curing condition (temp./time)                                                                        155° C./1 hr                                    volume resistivity (Ω · cm)                                                           6.5 × 10.sup.-4                                  Heat conductivity (cal/cm · sec. °C.)                                                --                                                     Extracted impurity (Cl.sup.- |Na.sup.+)                                                     10|5                                          ______________________________________                                    

                  TABLE 16                                                        ______________________________________                                        (C-940 AXLC manufactured by Amicon Corp.)                                     ______________________________________                                        State (Compounding ratio)                                                                            one-liquid type                                        Solvent                available                                              Composition (filler/resin)                                                                           Ag/polyimide                                           Curing condition (temp./time)                                                                        175° C./30 min˜                                                  275° C./30 min                                  volume resistivity (Ω · cm)                                                           --                                                     Heat conductivity (cal/cm · sec. °C.)                                                --                                                     Extracted impurity (Cl.sup.- |Na.sup.+)                                                     10|20                                         ______________________________________                                    

In multi-chip type image sensors, integrated circuit image sensor chipsmust be held on the base so that the distance between adjacent lightreceiving elements provided in adjacent integrated circuit image sensorchips is identical to the distance between adjacent light receivingelements within the same sensor chip. Unless the distance satisfies theabove-mentioned condition, the continuity of arrangement of lightreceiving elements is disturbed and therefore the reading quality isdegraded.

As described in the aforementioned paper, the chip mounting methods byuse of die bonding present a disadvantage that a positional deviation orerror (misregistration) of sensor chips occurs during heat curing (diebonding). The positional deviation of sensor chips causes an error inthe pixel pitch between neighboring light receiving elements of adjacentsensor chips (pixel pitch). The caused error deteriorates the readingquality.

The positional error of sensor chips caused during die bonding isfurther described. The thermal expansion coefficient of a sensor chipformed of silicon, α_(Si) is approximately 3.5×10⁻⁶ [1/°C.], and thethermal expansion coefficient of a base formed of alumina α_(B) =α(Al₂O₃) is nearly equal to 6.5×10⁻⁶ [1/°C.]. Curing temperatures of theaforementioned adhesives for use in die bonding are higher than 100° C.When sensor chips are fixed on a base by the heat curing process, thesensor chips and the base expand. However, it is noted that the degreeof expansion for the silicon sensor chips is different from (less than)the degree of expansion of the alumina base. After heating the assembledof the sensor chips and the base to the curing temperature, temperatureis decreased to room temperature. During this process, sensor chipscontracts and therefore the size of sensor chips at room temperaturebecomes smaller than that of those before heat curing. As a result, thepixel distance between adjacent sensor chips becomes wider than thatobtained before heat curing. In other words, the distance of adjacentelements within the same sensor chip becomes smaller than that obtainedbefore heat curing. Particularly, sensor chips used for the full-sizecontact type image sensor are long and narrow. Therefore, themisregistration between each sensor chip and the base resulting from adifference in thermal expansion coefficient occurs greatly. For afull-size contact type color image sensor, a deviation of the tone ofcolor occurs at joint portions of adjacent sensor chips. This causes agreat deterioration in image quality.

The aforementioned paper teaches that a conventional image sensor has amisregistration equal to ±15 μm. Such a misregistration cannot satisfyspecifications required for high-density image sensors, e.g., 400 dpi(dots per inch) for A3 size.

SUMMARY OF THE INVENTION

Accordingly, a general object of the present invention is to provide amulti-chip type image sensor in which the aforementioned disadvantagesof conventional sensors are eliminated.

A more specific object of the present invention is to provide amulti-chip type image sensor having a desired pixel pitch regulated withhigh precision.

The above objects of the present invention can be achieved by amulti-chip type image sensor comprising a base, a plurality of sensorchips arranged on the base so as to be staggered so that adjacent sensorchips among the sensor chips partially overlap with each other in thedirection perpendicular to a direction in which the sensor chips arearranged, the sensor chips being fastened to the base by an adhesive foruse in die bonding, each of the sensor chips having a plurality of lightreceiving elements arranged into a line at a predetermined pixel pitchPd, each of the sensor chip comprising silicon, and terminal pinssupported on the base and electrically coupled to the sensor chips. Thefollowing formula is satisfied:

    Pc=Pd×[1+(Tc-Ta)(α.sub.B -α.sub.Si)]

where Tc [°C.] is a curing temperature of the adhesive, Ta [°C.] is anenviroment temperature at which the image sensor is to be used, α_(Si)is a thermal expansion coefficient of each of the sensor chipscomprising silicon, α_(B) is a thermal expansion coefficient of thebase, and Pc is a pixel pitch at which the light receiving elements arearranged in each of the sensor chips before the adhesive is cured by diebonding.

The aforementioned objects of the present invention can also be achievedby a multi-chip type image sensor comprising a base, a plurality ofsensor chips arranged into a line on the base, the sensor chips beingfastened to the base by a die bonding process in which a first adhesiveis used, each of sensor chips comprising silicon and a plurality oflight receiving elements arranged into a line at a predetermined pitch,a glass plate mounted on the sensor chips so as to cover the lightreceiving elements, the glass plate being fastened to the sensor chipsby a second adhesive which hardens at a temperature lower than a curingtemperature of the first adhesive, and terminal pins supported on thebase and electrically coupled to the sensor chips. The followingcondition is satisfied:

    |α.sub.3 -α.sub.1 |<|α.sub.2 -α.sub.1 |

where α₁ is a thermal expansion coefficient of the base, α₂ is a thermalexpansion coefficient of each of the sensor chips, and α₃ is a thermalexpansion coefficient of the glass plate.

Another object of the present invention is to provide a method ofproducing the above-mentioned image sensor.

This object of the present invention can achieved by a method ofproducing a multi-chip image sensor including a plurality of sensorchips mounted on a base, each of the sensor chips having a plurality oflight receiving elements, comprising the steps of joining the pluralityof sensor chips together by a first adhesive, coating a second adhesiveon the base, the first adhesive hardening at a temperature lower than acuring temperature of the second adhesive; positioning the joined sensorchips on the base through the second adhesive, and heating the secondadhesive so that the image sensors are fastened to the base by the curedsecond adhesive.

Additional objects, features and advantages of the present inventionwill become apparent from the following detailed description when readin conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a plan view of a first embodiment of the present invention;

FIG. 1B is a front view of the first embodiment;

FIG. 1C is a side view of the first embodiment;

FIG. 2A is a plan view illustrating how to join adjacent sensor chipstogether;

FIG. 2B is a cross sectional view illustrating how to join adjacentsensor chips together;

FIG. 3 is a cross sectional view of a second embodiment of the presentinvention;

FIGS. 4 and 5 are views illustrating steps of constructing the imagesensor of the second embodiment;

FIG. 6 is a cross sectional view of a variation of the second embodimentshown in FIGS. 3 through 5;

FIG. 7 is a plan view of a third embodiment of the present invention;

FIGS. 8A and 8B are plan and cross sectional views of the thirdembodiment, respectively;

FIGS. 9A and 9B are plan and cross sectional views of a fourthembodiment of the present invention, respectively;

FIGS. 10A and 10B are plan and cross sectional views of a fifthembodiment of the present invention, respectively;

FIGS. 11A and 11B are plan and cross sectional views of a sixthembodiment of the present invention, respectively;

FIG. 11C is a diagram illustrating adhesive areas for the sixthembodiment; and

FIGS. 12A and 12B are diagrams illustrating adhesive areas for a seventhembodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A description is given of a multi-chip type image sensor according to afirst preferred embodiment of the present invention with reference toFIGS. 1A, 1B, 2A, 2B and 2C.

A multi-chip type image sensor 1 is constructed as follows. A pluralityof integrated circuit sensor chips (five sensor chips 3a, 3b, 3c, 3d and3e for the illustrated example) are mounted on a reading line defined ona long and narrow base 2 in such a manner that adjacent sensor chipssuch as chips 3a and 3b overlap slightly in the direction perpendicularto the reading line. In other words, the sensor chips 3a-3e arestaggered. Each of the sensor chips 3a-3e includes charge coupleddevices (CCDs). Read effective lengths l of the sensor chips 3a-3e aremutually identical. A read effective length of the image sensor L isequal to the sum of the read effective lengths l of the sensor chips3a-3e, that is, 5 l. The read effective length L is set equal to 304.8mm, for example. A transparent cover glass 4 is mounted on the base 2 soas to cover the arrangement of the sensor chips 3a-3e. A plurality ofpins 5 coupled to the related sensor chips 3a-3e which extendelevationally are arranged along a long end of the base 2. The sensorchips 3a-3e are electrically coupled to a printed circuit formed on thebase 2 in the same manner as conventional image sensors.

Each of the sensor chips 3a-3e is formed as shown in FIGS. 2A and 2B.Each of the sensor chips 3a-3e is made up of N (N is an integer) lightreceiving elements S₁, S₂, . . . , S_(N) which are arranged into a lineat a predetermined pitch P measured in the longitudinal direction. Theadjacent sensor chips 3a and 3b are located so that the last lightreceiving element S_(N) of the sensor chip 3a and the first lightreceiving element S₁ of the sensor chip 3b are arranged at the pitch P.The last light receiving element S_(N) of the sensor chip 3b and thefirst light receiving element S₁ of the sensor chip 3c are arranged atthe pitch P. The other sensor chips are located in the same manner. Thesensor chips 3a-3e are fastened on the base 2 by use of an adhesive 6for use in die bonding, as shown in FIG. 2B.

The base 2 is formed of alumina, and each of the sensor chips 3a-3e isformed of silicon. Parameters indicative of properties of alumina areshown below.

Major component: Al₂ O₃ 92-94%, SiO₂, MgO, etc.

Resistance: 10¹² [Ω·cm]

Thermal expansion coefficient (0°-100° C.): 6.0-6.5×10⁻⁶ [°C⁻¹ ]

Thermal conductivity: 0.03-0.04 [cal/cm·s·°C.]

Permittivity (1 MHz): 8.5-9.5

Dielectric dissipation factor (1 MHz): 0.001

Heat-resistant temperature: 1000 [°C.] or higher

Modulus of longitudinal elasticity: 3.7-4.1×10⁴ [kg/mm² ]

Bending strength (tensile strength): 30-35 [kg/mm² ]

α-ray occurrence ratio: 0.03-0.5 [number/time·cm² ]

Parameters indicative of properties of silicon are shown below.

Major component: Si

Resistance: 2.3×10⁴ [Ω·cm]

Thermal expansion coefficient (0°-100° C.): 3.5×10⁻⁶ [°C.⁻¹ ]

Thermal conductivity: 0.2-0.35 [cal/cm·s·°C.]

Permittivity (1 MHz): 12

Dielectric dissipation factor (1 MHz): None

Heat-resistant temperature: 1414 [°C.]

Modulus of longitudinal elasticity: C₁ 1.7, C₁₂ 0.7, C₁₄ 0.8 [kg/mm² ]

Bending strength (tensile strength): 30 [kg/mm² ]/10 min

α-ray occurrence ratio: 0.01 [number/time·cm² ]

For example, the aforementioned EPO-TEK H-20ELC (manufactured by EpoxyTechnology Corp.) is used as the adhesive 6 for use in die bonding. Thecuring temperature Tc is set equal to 120° C. The enviroment temperatureTa to be set during chip production (particularly, exposure for maskalignment) and use of the image sensor 1 is equal to 20° C.

Under the above-mentioned condition, a difference in expansion(difference in size) obtained at the curing temperature Tc is written asfollows: ##EQU1## where l(Al₂ O₃) is the length of the alumina base 2,l_(Si) is the length of each of silicon sensor chips 3a-3e (chipeffective length), α(Al₂ O₃) is the thermal expansion coefficient of thealumina base 2, and α_(Si) is the thermal expansion coefficient of eachof the silicon sensor chips 3a-3e. Hereinafter, the silicon sensor chips3a-3e are also referred to as silicon sensor chips 3 for convenience'sake. In the state defined by the above formula, the sensor chips 3 arefastened on the base 2. On the other hand, the alumina base 2 and thesilicon sensor chips 3 are contracted as the temperature thereofdecreases from the curing temperature Tc to room temperature Ta at whichthe image sensor 1 is used. It is noted that the thermal expansioncoefficient for alumina is larger than that for silicon and thereforethe alumina base 2 is contracted more greatly than the silicon sensorchips 3. During contraction, a strain ε is caused to equilibrate force Pexerted on the alumina base 2 and the sensor chips 3 so that the sensorchips 3 are stabilized on the alumina chip 2. The strain ε is written asfollows:

    ε=σ/E

σ=P/A

where σ is a stress, E is the modulus of longitudinal elasticity, and Ais a cross section. Consequently, the equilibrium condition is describedas follows: ##EQU2## and the following is obtained:

    ε(Al.sub.2 O.sub.3)/ε.sub.Si =(A.sub.Si ·E.sub.Si)/[A(Al.sub.2 O.sub.3)·E(Al.sub.2 O.sub.3)]<<1

For example, A_(Si) =0.7 mm wide×0.5 mm thick=0.35 mm², A(Al₂ O₃)=20 mmwide×1.6 mm thick=32 mm², E_(Si) ≈1.7×10⁴ kg/mm², and E(Al₂ O₃)≈3.8×10⁴kg/mm². As a result, and stress σ is less dependent on the strain ε.That is, the alumina base 2 is contracted according to its thermalexpansion coefficient α(Al₂ O₃), and the sensor chip 3 is compressedwhere a strain corresponding to the difference in thermal expansioncoefficient between the alumina base 2 and each sensor chip 3, that is,α(Al₂ O₃) -α_(Si) is caused. For example, when l=61 mm, and (Tc-Ta)=95°C., an amount of strain caused in each sensor chip 3 is described asfollows: ##EQU3## The above value corresponds to the misregistration ofpixel arrangement caused at joint portions of adjacent sensor chips.

On the other hand, the present embodiment is made by paying attention tothe fact that each of the sensor chips 3 is adhesive-fastened to thebase 2 by the heat curing (heat hardening) process and by taking intoconsideration thermal expansion and contraction due to heat curing. Thatis, according to the embodiment, a pixel pitch Pc before heat curing isset larger than the predetermined pixel pitch Pd so that the compressionstrain caused in each sensor chip 3 when temperature thereof returns toroom (enviroment) temperature Ta from the curing temperature Tc, iscompensated beforehand. In detail, each of the sensor chips 3 isproduced so that the pixel pitch Pc is selected according to thefollowing formula:

    Pc=Pd×[1+(Tc-Ta)(α(Al.sub.2 O.sub.3)-α.sub.Si)]

where Tc [°C.] is the curing temperature of the adhesive 6 for heatcuring die bonding, Ta [°C.] is room temperature set equal to enviromenttemperature of the image sensor 1 when it is used, α_(Si) is the thermalexpansion coefficient of the sensor chips 3 formed of silicon, and α(Al₂O₃) is the thermal expansion coefficient of the base 2 formed ofalumina.

The above-identified formula can be written into a general form asfollows:

    Pc=Pd×[1+(Tc-Ta)(α.sub.B -α.sub.Si)]

where α_(B) is the thermal expansion coefficient of a material formingthe base 2.

It is particularly effective to producing the sensor chips 3 so thatσ≧Pd/4 is satisfied because the CCITT defines that reading error shouldbe within ±25% of the regular pitch Pd. This requirement is alsoimportant in view of image reading quality. For a color sensor chipwhere a color filter is provided in the direction of arrangement of thesensor chips 3, σ≧Pd/12 should be satisfied in order to reduce a changein the tone of color.

The aforementioned embodiment relates to the image sensor which uses theconductive adhesive 6 for use in die bonding by resin bonding. Thepresent invention includes an image sensor which uses a nonconductiveadhesive for die bonding, or die bonding processes other than resinbonding. In eutectic bonding, solder bonding or glass bonding,soldifying temperature of an adhesive used is equal to the curingtemperature for resin bonding.

A description is given of a second preferred embodiment of the presentinvention with reference to FIGS. 3 to 5. Referring to these figures, aplurality of integrated circuit image sensor chips 13a, 13b and 13c eachincluding a plurality of CCDs are arranged into a line and fixed on abase 11 through an adhesive 12 for use in die bonding. The sensor chips13a and 13b are separated from each other at a predetermined interval,and the sensor chips 13b and 13c are separated from each other at thesame interval. The sensor chips 13a, 13b and 13c are electricallycoupled to a printed circuit (not shown) formed on the base 11. Each ofthe sensor chips 13a, 13b and 13c is formed of silicon, and the base 11is formed of alumina, for example. As shown in FIG. 3, a stationarytransparent glass plate 17 which is narrower and long as the base 11 ismounted on and fastened to the sensor chips 13a, 13b and 13c through atransparent adhesive 18 of a room temperature hardening type or anultraviolet hardening adhesive. Therminal pins such as the terminal pins5 shown in FIG. 1C are omitted for the sake of simplicity.

In an assembling process, as shown in FIG. 4, the sensor chips 13a, 13band 13c are arranged into a line on the glass plate 17, and fastenedthereto by irradiating ultraviolet rays onto the adhesive 18 at roomtemperature. The surface of the glass plate 17 on which the sensor chips13a, 13b and 13c are mounted is opposite to the surface thereof whichfaces the base 11. Then, as shown in FIG. 5, spacers 14 are fastened onthe base 11 through an adhesive 15 of a room temperature hardening type.The spacers 14 are formed so as to completely or partially surround thesensor chips 13a, 13b and 13b. In the illustrated structure, the spacers14 are provided on both the sides of the arrangement of the sensor chips13a, 13b and 13c in the longitudinal direction. Thereafter, the adhesive12 is coated on the base 11. Then, as shown in FIG. 3, the assemblyshown in FIG. 4 is positioned on the adhesive 12 so that the bottomsurface of each of the sensor chips 13a, 13b and 13c faces the base 11.Then the assembly shown in FIG. 5 is subjected to the heat curingprocess so that the sensor chips 13a, 13b and 13c are adhesive-fastenedto the base 11. Finally, a transparent sealing glass 16 is mounted onthe spacers 14 and fastened thereto through an adhesive layer 20 of aroom temperature hardening type.

The glass plate 17 is formed by a Na₂ O-CaO-SiO₂ -based glass plate. Thethermal expansion coefficient α_(SH) and the modulus of longitudinalelasticity of the Na₂ O-CaO-SiO₂ -based glass plate 17 E_(SH) are asfollows:

    α.sub.SH ≈8.5×10.sup.-6 [°C.]

    E.sub.SH ≈0.7×10.sup.4 [kg/mm.sup.2 ]

For these values, an amount of expansion of the Na₂ O-CaO-SiO₂ -basedglass plate 17 caused during heat curing is obtained as follows:

    ε.sub.SH /ε.sub.Si <<1

where it is assumed that other parameters are the same as thosedescribed previously. Therefore, the following formula is obtained:##EQU4## An amount of expansion of the alumina base 11 obtained at thistime is 40 μm as described previously. It follows that the difference inexpansion between the glass plate 17 and the base 11 is fixed to 8 μm.As a result, the following relationship is obtained:

    ε.sub.SH /ε(Al.sub.2 O.sub.3) >1

It can be seen that the positional error of adjacent sensor chips causedduring the step in which temperature thereof decreases to roomtemperature from the curing temperature is equal to or less than 12 μm.This is considerably less than that for the conventional image sensor.

It can be seen from the above description that the glass plate 17 shouldbe formed of a material which satisfies the following condition:

    |α.sub.G -α.sub.B |<|α.sub.Si -α.sub.B |

where α_(G) is the thermal expansion coefficient of a material formingthe glass plate 17.

A description is given of a variation of the second embodiment withreference to FIG. 6. An essential feature of the variation is that aglass plate 17a functions to not only fix the sensor chips 13a, 13b and13c but also seal them. In other words, the glass plate 17a shown inFIG. 6 serves as both the glass plate 17 and the sealing glass 16 shownin FIG. 5. The glass plate 17a is mounted on and fastened to the spacer14 through the adhesive 15.

The variation shown in FIG. 6 presents the following specificadvantages, as compared with the second embodiment shown in FIGS. 3through 5. First, the image sensor of the variation can be formed by asmaller number of structural parts than the image sensor of the secondembodiment. Second, loss of light for the variation is less than thatfor the second embodiment.

The aforementioned second embodiment and its variation relate to theimage sensors which use the conductive adhesive 18 for use in diebonding by resin bonding. The present invention includes an image sensorwhich uses a nonconductive adhesive for die bonding or die bondingprocesses other than resin bonding. In eutectic bonding, solder bondingor glass bonding, soldifying temperature of an adhesive used is equal tocuring temperature for resin bonding.

A description is given of a third preferred embodiment of the presentinvention with reference to FIGS. 7, 8A and 8B. FIG. 7 illustrates theentire structure of an image sensor according to the third embodiment.The illustrated image sensor includes a plurality of integrated circuitimage sensor chips 32a, 32b, 32c and 32d arranged into a line on a base34. The illustrated arrangement itself is conventional. An essentialfeature of the third embodiment exists in a process of constructing theimage sensor. The sensor chips 32a-33d are adhesive-joined together soas to be arranged in a line in such a manner that each joint portionbetween each set of two adjacent sensor chips among the sensor chips32a-32d is provided with an adhesive 36 which is hardened attemperatures lower than a temperature at which an adhesive 33 for use ina heat curing type die bonding is cured. Then, the joined sensor chips32a-32d are mounted on the base 34 through the adhesive 33 and fixedthereto by the curing process.

A low-temperature hardening type adhesive or an ultraviolet hardeningtype adhesive may be used as the adhesive 36. For example, type "390" ofthe heat-resistant adhesive 300 series (hardening condition: 30 minutesat room temperature) manufactured by Epoxy Technology Corp. orheat-resistant adhesive series 301 (adhesive for optical use) (hardeningcondition: one hour at 65° C.) manufactured by the same company is alow-temperature hardening type adhesive. NOR60 manufactured by NorlandProducts Corp is an ultraviolet hardening type adhesive. The adhesive 33may be one of the aforementioned heat curing type adhesive agents fordie bonding shown in Tables 5 through 16.

The occurrence of positional errors (misregistration) of the adjacentsensor chips can be suppressed since the joint portions 35 are adhesiveconnected through the adhesive 36 at low temperatures. The joined sensorchips 32a-32d having less positional errors are fixed on the base 33 byuse of the heat curing adhesive 33 for use in die bonding. Thus, itbecomes possible to suppress the occurrence of misregistration betweenthe sensor chips 32a-32d and the base 33 resulting from a difference inthermal expansion coefficient therebetween. Hence, it is possible toprecisely define the pitch between adjacent light receiving elements inadjacent sensor chips so that it is almost the same as chip alignmentprecision (5 μm or less, for example). Thus, it is possible to preventdegradation of reading quality of light receiving elements which arelocated in the vicinity of the joint portions 35. As a result, the thirdembodiment can provide a multi-chip image sensor having a high densityof 400 dpi.

A description is given of a fourth preferred embodiment of the presentinvention with reference to FIGS. 9A and 9B, in which those parts whichare the same as those or the same properties as those shown in FIGS. 7,8A and 8B are given the same reference numerals. The adhesive 33 for usein heat curing type die bonding is coated on the surface of the base 34except an area corresponding to the joint portion 35. Next, the adhesive36 is dispensed on the area corresponding to the joint portion 35. Thenthe sensor chips 32a-32d are aligned on the adhesive 33 and the adhesive36 so that the joint portion 35 is located on the layer of the adhesive36. Then, the adhesive 36 is hardened so that the sensor chips 32a-32dare adhesive-joined together on the base 34. Thereafter, the adhesive 33for use in heat curing type die bonding is cured, so that the sensorchips 32a-32d can rigidly be fixed to the base 34.

A description is given of a fifth preferred embodiment of the presentinvention with reference to FIGS. 10A and 10B. The production methodaccording to the fifth embodiment can be applied when constructing theimage sensor of the first embodiment. First, the sensor chips 32a and32b are located so as to be staggered and joined together by theadhesive 36. Of course, the other sensor chips are processed at the sametime. Then the staggered sensor chips 32a-32d are mounted on the base 34through the adhesive 33 and is then cured.

A description is given of a sixth preferred embodiment of the presentinvention with reference to FIGS. 11A through 11C. An essential featureof the sixth embodiment is that the aforementioned adhesive 36 isprovided in an intermediate surface area of the base 34 interposedbetween the joint portions 35. For the embodiment shown in FIGS. 11Athrough 11B, each of the adhesive layers 36 is located in an areacorresponding to the center of the corresponding sensor chip whenmounted on the base 34. As described previously, the low-temperaturehardening type adhesive 36 hardens at a temperature lower than theadhesive 33 for use in heat curing type die bonding. The adhesive 33 isalso coated in surface areas of the base 34 in which the sensor chips32a-32d are to be positioned. Then, the sensor chips 32a-32d arepositioned on the adhesive layers 33 and 36 and pressed against the base34 at room temperature. Thereby, the sensor chips 32a-32d areprovisionally fixed to the base 34 and positioned thereon. Thereafter,the heat curing type adhesive 33 for use in die bonding is cured byheating so that the sensor chips 32a-32d can tightly be adhesive-fixedto the base 34. The provisional fixing of the sensor chips 32a-32d byuse of the adhesive layer 36 functions to prevent the occurrence ofmisregistration of the sensor chips 32a-32d arising from the differencein thermal expansion coefficient between the sensor chips 32a-32d andthe base 34. Thus, it is possible to precisely define the pitch betweenadjacent light receiving elements in adjacent sensor chips so that it isalmost the same as chip alignment precision (5 μm or less, for example).

The adhesive 36 of a low-temperature hardening type may be one of theadhesives described previously. Alternatively, "ECOBOND" manufactured byGrace Japan may be used as the adhesive 6.

A description is given of a seventh embodiment of the present inventionwith reference to FIGS. 12A and 12B. An essential feature of the seventhembodiment is that at least two adhesive points by use of thelow-temperature hardening type adhesive 36 are provided for each of thechips 32a-32d so that they are not adjacent to each other. As shown inFIG. 12A, the adhesive spots 33 and the adhesive spots 36 are coated onthe base 34 by the dispense process. Then, the sensor chips 32a-32d arepositioned and pressed against the base 34. Thereby, as shown in FIG.12B, the adhesive spots 33 and 36 spread on the entire interface betweenthe base 34 and each of the sensor chips 32a-32d. The sensor chips32a-32d are provisionally fixed on the base 34 by the adhesive 36 atroom temperature. Then the adhesive 33 is cured whereby the sensor chips32a-32d are completely adhesive-fastened to the base 34. The seventhembodiment is particularly advantageous to an image senor which includesvery long sensor chips (0.7 mm wide× 70 mm long, for example). In theillustrated seventh embodiment, the adhesive spots 33 and 36 arearranged into a line. Alternatively, the adhesive spots 33 and 36 may bestaggered.

The present invention is not limited to the aforementioned embodiments,and variations and modifications may be made without departing from thescope of the present invention.

What is claimed is:
 1. A multi-chip type image sensor comprising:a base;a plurality of sensor chips arranged on said base so as to be staggeredso that adjacent sensor chips among said sensor chips partially overlapwith each other in the direction perpendicular to a direction in whichsaid sensor chips are arranged, said sensor chips being fastened to saidbase by an adhesive for use in die bonding, each of said sensor chipshaving a plurality of light receiving elements arranged into a line at apredetermined pixel pitch Pd, each of said sensor chip comprisingsilicon; and terminal pins supported on said base and electricallycoupled to said sensor chips, the following formula being satisfied:

    Pc=Pd×[1+(Tc-Ta)(α.sub.B -α.sub.Si)]

where Tc [°C.] is a curing temperature of said adhesive, Ta [°C.] is anenviroment temperature at which said image sensor is to be used, α_(Si)is a thermal expansion coefficient of each of said sensor chipscomprising silicon, α_(B) is a thermal expansion coefficient of saidbase, and Pc is a pixel pitch at which said light receiving elements arearranged in each of said sensor chips before said adhesive is cured bydie bonding.
 2. A multi-chip type image sensor as claimed in claim 1,wherein said base comprises alumina.
 3. A multi-chip type image sensoras claimed in claim 1, wherein said adhesive is electrically conductive.4. A multi-chip type image sensor as claimed in claim 1, furthercomprising a cover glass which is mounted on said base so as to coversaid sensor chips.